This invention relates to purified natural glass products having low concentrations of soluble substances. More particularly, this invention relates to purified natural glass products having low slurry electrical conductivities (i.e., less than about 18 .mu.S-cm.sup.-1). Preferred embodiments are further characterized by low concentrations of soluble iron (i.e., less than about 2 mg Fe/kg product) and/or low concentrations of soluble aluminum (i.e., less than about 10 mg Al/kg product). These products may be prepared from natural glasses and natural glass products, including, for example, expanded perlite, pumice, expanded pumice, and volcanic ash. The products of the present invention retain the intricate and porous characteristics of the feed material but possess low concentrations of soluble substances, thereby permitting much greater utility, particularly in filtration applications.